Think of it this way: The sole reason to shift from a 40-front formation to a 30-front formation is to have a "floating" blitzer, instead of a "predictable" pressure.
With only 3 defensive linemen, the Defensive Coordinator have a floating piece behind the line. He can send either outside linebacker in a blitz, or even one of the two inside ones.
He can do twists, making the inside LB exchange places with the outside LB just after the snap, to have a better run defense on the outside (with the ILB) and rush from inside. The whole concept of the Zone Blitz (today mainly run by Green Bay's Dom Capers and Pittsburg's Dick LeBeau) feed on the floating backer concept.
But, in order to do so, the 30-front line must be heavier and capable of attacking two gaps. The NT and the 30-front DE must be strong men able to contain the O-Line, instead of just shoot thru gaps. Guys like Dwight Freeney or Jared Allen are speed rushers that avoid the line completely, and not fit for a 30-front defense.
I believe the best example of that is the mid-2000's Steelers line, with Aaron Smith, Casey Hampton and Brett Keisel. None of them have "cool" stats, like sacks or tackles for loss. But was a sure as hell great line.
And when I say "predictable" because, conceptually, a 40-front line will attack the offensive line gaps and thus try to collapse the pocket. In a traditional 4-3 system, the LBs usually do exactly what their name says, the back-up the line for missing running assigments, screen plays, short routes. The job to get to the QB is solely up to the line (specially the ends) --- this is not absolutely truth, of course; a 40-front defense can send LBs to rush the passer in expense of cover ability. In 2013, was pretty common to see 4-2-5 Nickel defenses sending the nickel back (the third DB) on a blitz.
